The use of hollow motors and hydraulic pistons to drive and rotate plasticating screws is known. However, none of the known systems suggests combining the advantages of hollow motors to rotate the plasticating screw while using a hydraulic piston to move it lengthwise.
U.S. Pat. No. 4,105,147 to Stubbe describes a screw extruder rotated by a gear drive from an electric motor and moved lengthwise by a hydraulic piston. The screw has a splined shaft end to permit sliding of the shaft through the gear drive.
The U.S. Pat. No. 4,895,505 to Fanuc Ltd. describes a linear motor for moving an injection screw linearly. The linear motor includes a series of permanent magnets attached to the motor armature that react with the alternating current supplied to the surrounding stator windings to cause linear movement of the armature and the screw shaft attached to the armature. The patent describes the use of a hollow motor to move a screw shaft linearly.
The U.S. Pat. No. 5,540,495 issued Jul. 30, 1996 to Krauss-Maffei describes an extruder screw drive that includes a first motor for translating movement of the screw and a second motor for rotating the screw. The described embodiment shows two hollow motors. The drive means for translating the screw and the slide means for rotating the screw fit partially within one another.
U.S. Pat. No. 5,645,868 to Reinhart describes a drive apparatus for an injection unit that includes a hollow electric motor that engages the screw shaft through three clutches. One clutch provides rotation of the screw, a second enables forward movement of the screw and a third prevents the screw from rotating while it is being moved forward. No hydraulic units are used.
U.S. Pat. No. 5,747,076 to Jaroschek et al describes an injection-molding machine that uses a hydraulic piston to assist an electric motor driving a rack and pinion mechanism to advance the screw.
The U.S. Pat. No. 5,804,224 issued Sep. 8, 1998 to Fanuc Ltd. describes an arrangement where a ball screw is integrally formed on the rotor shaft. A motor positioned coaxially with it rotates the ball screw.
The U.S. Pat. No. 5,891,485 issued Apr. 6, 1999 to Sumitomo describes an injection apparatus that includes two hollow shaft electric motors. One motor is intended to rotate the screw shaft while the other moves it lengthwise. The rotors of the two motors are coupled to the shaft. Each rotor is located in a separate chamber.
U.S. Pat. No. 6,068,810 to Kestle et al describes an injection unit having a quill inside a piston to enable retraction and extension of the screw by the application of hydraulic pressure. A motor rotates the quill, which is connected to the piston through a spline to thereby rotate the screw. The motor attaches to the end of the quill.
U.S. Pat. No. 6,108,587 to Shearer et al describes an injection molding system that includes a motor for driving gears to rotate the screw and a hydraulic piston for translating the screw.
U.S. Pat. No. 6,478,572 to Schad describes an injection unit that uses a single electric motor to rotate an extruder screw and charge a hydraulic accumulator. The charge in the accumulator is directed to stroke the extruder screw.
U.S. Pat. No. 6,499,989 describes a device for removing disks from a mold. In the described embodiments a hollow electric motor is used to rotate the take-out shaft and a linear electric motor is used to move the shaft linearly. The hollow motor drives the shaft through a gearbox that enables the speed of the shaft to be varied. As an alternative, the patent suggests that a pneumatic or hydraulic cylinder could be used to move the shaft linearly. In the embodiments described, the linear actuator is located outside the rotary actuator. This provides an assembly that is larger and less cost effective.
U.S. Pat. No. 6,517,336 to Emoto et al and European Patent No. 0967064 A1 to Emoto disclose an injection molding system having a hollow electric motor that rotates a screw shaft and at the same time causes the shaft to advance by means of a connection to a ball screw shaft/spline shaft unit. A separate metering motor rotates the screw to load the screw with resin. Rotational movement is provided through a belt and pulley arrangement that can rotate the screw independently of the rotor on the hollow motor. The rotor on the hollow motor is attached to a splined portion of the screw shaft and is used to rotate the splined portion, which, in turn, rotates a ball screw to drive a ball nut and thereby move the shaft lengthwise.
U.S. Pat. No. 6,530,774 to Emoto describes an injection molding system using an electric motor and gear train to rotate the screw and a hollow shaft electric motor to move the screw lengthwise by driving a ball screw shaft through a splined shaft connection.
U.S. patent application Ser. No. 2002/0168445 A1 to Emoto et al describes an injection system that also includes a metering motor and a hollow shaft motor to rotate the screw and move the screw lengthwise, respectively.
The European Patent application 1162053 published Dec. 12, 2001 to Krauss-Maffei describes a two motor system where one motor provides rotational movement of the screw shaft and the other motor provides translational movement of the screw shaft. Clutch arrangements are used to enable the motors to operate separately or together.
The Japanese Patent 61266218 published Nov. 25, 1986 to Sumitomo describes a two motor injection system using hollow motors, a ball drive mechanism and splined shafts.
While these references describe many combinations of electric and hydraulic driving systems for a screw of an injection-molding machine, they fail to describe a system combining the unique advantages of better control of the positioning of the screw with a hollow electric motor and the high injection power provided by a hydraulic injection unit. The present invention provides a compact injection unit having the unique advantages of both electric and hydraulic driving systems.